Nuclear Magnetic Resonance Study of Water in Gluten in the Glassy and Rubbery State

Cereal Chem. 73(5):618-624 Water mobility in hydrated gluten at 0-50% mc was studied using 170 sorption isotherm. Freezable water was only observed at >18% mc, when and H nuclear magnetic resonance (NMR). Glass transition behavior the sample was in the rubbery state (beyond the midpoint glass measured by dynamic mechanical analyzer (DMA) and by differential transition) and water was unfreezable in the glassy region. Results from scanning calorimetry (DSC) was compared to the NMR signal intensity. the different techniques (NMR, DMA, DSC, and sorption isotherm) The H NMR signal intensity increased during glassy-rubbery transition showed good correlation, although experimental conditions, sample due to hydration. 170 NMR detected the signal when >0.21 g of water/g preparations, and the time frame of each experiment were inherently total occurred in the "free" or bulk water region, as determined from the different. Hydration of wheat protein (gluten) greatly influences the mechanical and rheological properties of a dough (Finney and Shogren 1972, Mani et al 1992). The mobility of water is critical for the interaction of gluten and water to form a viscoelastic network. As a low molecular weight diluent, water acts as a plasticizer and increases the mobility of the system (Ferry 1980). It affects the functional properties by causing conformational changes allowing for hydrophobic interaction (Kinsella and Hale 1984) and also acts as a solvent for the hydrophilic and watersoluble components (low molecular weight gluten proteins) at low ionic strength. Hydration with mixing causes the unraveling and unfolding of the tightly packed aggregated gluten proteins and the formation of a viscoelastic network that contributes to optimum dough volume during baking (Hoseney and Rogers 1990). Gluten proteins are the principal proteins occurring in wheat. Based on their solubility, they are broadly divided into two categories: 1) gliadins (soluble in aqueous alcohol) and 2) glutenins (soluble in dilute acid) (Bushuk 1985). Gliadins are relatively short chain proteins (molecular weight ranging from 25,000 to 100,000 Da) and are responsible for the extensibility and cohesive properties of gluten. Glutenins have a higher molecular weight than gliadins (ranging from 100,000 to 1 million Da) and contribute to the elasticity of gluten (Redman 1971). The glassy-rubbery behavior of amorphous and partly crystalline foods has become an area of increasing interest (Noel et al 1990, Slade and Levine 1993). The glassy-rubbery behavior of gluten has already been reported (Hoseney et al 1986, Kalichevsky et al 1992a). The glass transition (Tg) of gluten fractions (glutenin and gliadin) has also been studied using differential scanning calorimetry (DSC) and mechanical spectrometry. Cocero and Kokini (1991) and deGraff et al (1993) reported that gliadins had a lower Tg than did gluten and glutenin, which was attributed to the lower molecular weight and the lower concentration of hydrophilic amino acids. Kalichevsky et al (1992a,b) and Kalichevsky and Blanshard (1992) reported the effect of lipids, emulsifiers, and sugars on the glass transition behavior of gluten as a function of moisture content. The effect of plasticizers on the mechanical and water vapor permeability barrier properties of wheat gluten films was studied by Gontard et al (1993) and Cherian et al (1995). Water mobility has an important effect on the overall mobility and structural properties of metastable food polymer systems 'Department of Food Science, University of Massachusetts, Amherst, MA 01003. Phone: 413/545-2276. Fax: 413/545-1262 Publication no. C1996-0826-01 R. © 1996 American Association of Cereal Chemists, Inc. 618 CEREAL CHEMISTRY (Ablett and Lillford 1991). Nuclear magnetic resonance (NMR) techniques provide a powerful tool to study specific molecular interactions of water with other components. NMR has been widely used to examine water mobility by energizing chosen nuclei of water ('H, H, and 'TO) in various systems (Hills et al 1990, Belton et al 1991, Hills 1991). Details of the use of NMR to study water in foods can be found elsewhere (Richardson and Steinberg 1987, Belton 1990, Chinachoti and Stengle 1990). 'H NMR has been commonly used to determine water mobility in various food systems. However, a major drawback of using 'H NMR is the effect of the cross-relaxation process resulting in line broadening (Edzes and Samulski 1978, Shirley and Bryant 1982). The NMR data of a quadropolar nucleus (e.g., H) is not affected by cross-relaxation, but may show the effect of chemical exchange, making the interpretation of the results difficult (Richardson and Steinberg 1987, Kakalis and Baianu 1988). The problems encountered with 'H and H NMR can be overcome using 'IO NMR, as shown in the studies with sucrose (Chinachoti and Stengle 1990), lysozyme (Kakalis and Baianu 1988), and wheat flour (Richardson et al 1985). Being quadrupolar, 'IO nuclei does not exhibit any cross-relaxation and its exchange rate is extremely slow. Proton exchange broadening in 170 NMR can be easily eliminated by proton decoupling (Richardson and Steinberg 1987). 'H and H NMR were used to quantify the fractions of bound and free water occurring in wheat flour dough (Leung et al 1979, 1983; d'Avignon et al 1990). Attempts to quantitate the interaction of water with macromolecules using 'H, H, and '0 NMR have been extensive (Kakalis and Baianu 1988, Otting et al 1991). Studies using these nuclei to relate the molecular level changes of macromolecules on hydration are fraught with problems because of the model system dependence and the proton-proton cross polarization effect on NMR line broadening (d'Avignon et al 1990, Belton 1991). Richardson et al (1985) studied 170 NMR water mobility and rheological characteristics of wheat flour suspensions and observed no direct relationship between water mobility and rheological properties. However, the wheat flour-water suspensions studied were at high moisture content (60-90% wb), which was not within the moisture range where glass transition occurs. Belton et al (1988) applied Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence to study the proton transverse relaxation in dry gluten. There have been some studies on conformation of gluten and water structure using 13C NMR (Baianu 1981, Belton et al 1985, Ablett et al 1988). Water mobility in gluten using proton NMR was recently reported by Kalichevsky et al (1992a). Thermal techniques such as dynamic mechanical analysis (DMA) and DSC have been widely used to study glass transition behavior in various systems (Wunderlich 1990, Mathieson and Ibar 1991, Foreman et al 1992). For DMA, a stress is applied to the sample as a sinusoidal wave function with an increase in temperature. Depending on the nature of the material (viscoelastic character), the resulting strain frequency is either in phase with the stress (if the material is ideally elastic) or out of phase (if the material is viscoelastic). The extent to which the resulting strain is out of phase with the applied stress is given by the phase angle 5. The loss tangent (tan 5) is the ratio of E' (loss modulus) to E' (storage modulus). Change in E', E", and tan 5 has been used to characterize the transition behavior. In this study, the peak of the tan 5 dependence was used as the midpoint temperature (Tg). For the materials undergoing glass transition, changes observed in rheological properties are often greater than corresponding changes in thermal energy (Foreman et al 1992), and thus the DMA is usually more sensitive than the DSC. Unfortunately, little work has been done to try to relate the molecular mobility (as influenced by water in this case) in gluten in relation to such changes in thermomechanical properties. Such a relationship would be critical to a more comprehensive understanding of gluten functionality. Therefore, the objective of this study was to investigate thermomechanical properties of wheat gluten in association with mobility as observed by NMR upon hydration. MATERIALS AND METHODS Materials Wheat gluten from hard red spring wheat (Sigma Chemical Co., St. Louis, MO) with protein content of 80% db (N x 5.7) was washed with distilled water (1:3, w/v) and freeze-dried (Virtis Sublimator, model 50-SRC, Gardiner, NY). The sample was subsequently stored in a desiccant chamber (with phosphorous pentoxide) at room temperature. Deuterium oxide (99.9% pure) and 0.1% O-enriched water obtained from Cambridge Isotope Ltd. (CIL, Cambridge, MA) were used for the NMR study. Salts used for the preparation of saturated solutions were all analytical grade (Fisher Scientific Co., Fairlawn, NJ). Methods A broad outline of the experimental approach used in this study is shown in Figure 1. Differential Scanning Calorimetry A Seiko DSC 100 (Seiko Instruments, Inc., Torrance, CA) was used for studying the thermal transition behavior of gluten. Low moisture samples (<20% mc), were prepared by equilibration with saturated salt solution at various relative humidities (0-97% RH) B 03r